Malate-aspartate shuttle and exogenous NADH/cytochrome c electron transport pathway as two independent cytosolic reducing equivalent transfer systems.

In mammalian cells aerobic oxidation of glucose requires reducing equivalents produced in glycolytic phase to be channelled into the phosphorylating respiratory chain for the reduction of molecular oxygen. Data never presented before show that the oxidation rate of exogenous NADH supported by the malate-aspartate shuttle system (reconstituted in vitro with isolated liver mitochondria) is comparable to the rate obtained on activation of the cytosolic NADH/cytochrome c electron transport pathway. The activities of these two reducing equivalent transport systems are independent of each other and additive. NADH oxidation induced by the malate-aspartate shuttle is inhibited by aminooxyacetate and by rotenone and/or antimycin A, two inhibitors of the respiratory chain, while the NADH/cytochrome c system remains insensitive to all of them. The two systems may simultaneously or mutually operate in the transfer of reducing equivalents from the cytosol to inside the mitochondria. In previous reports we suggested that the NADH/cytochrome c system is expected to be functioning in apoptotic cells characterized by the presence of cytochrome c in the cytosol. As additional new finding the activity of reconstituted shuttle system is linked to the amount of α-ketoglutarate generated inside the mitochondria by glutamate dehydrogenase rather than by aspartate aminotransferase.

Valinomycin induced energy-dependent mitochondrial swelling, cytochrome c release, cytosolic NADH/cytochrome c oxidation and apoptosis.

In valinomycin induced stimulation of mitochondrial energy dependent reversible swelling, supported by succinate oxidation, cytochrome c (cyto-c) and sulfite oxidase (Sox) [both present in the mitochondrial intermembrane space (MIS)] are released outside. This effect can be observed at a valinomycin concentration as low as 1 nM. The rate of cytosolic NADH/cyto-c electron transport pathway is also greatly stimulated. The test on the permeability of mitochondrial outer membrane to exogenous cyto-c rules out the possibility that the increased rate of exogenous NADH oxidation could be ascribed either to extensively damaged or broken mitochondria. Accumulation of potassium inside the mitochondria, mediated by the highly specific ionophore valinomycin, promotes an increase in the volume of matrix (evidenced by swelling) and the interaction points between the two mitochondrial membranes are expected to increase. The data reported and those previously published are consistent with the view that "respiratory contact sites" are involved in the transfer of reducing equivalents from cytosol to inside the mitochondria both in the absence and the presence of valinomycin. Magnesium ions prevent at least in part the valinomycin effects. Rather than to the dissipation of membrane potential, the pro-apoptotic property of valinomycin can be ascribed to both the release of cyto-c from mitochondria to cytosol and the increased rate of cytosolic NADH coupled with an increased availability of energy in the form of glycolytic ATP, useful for the correct execution of apoptotic program.

Ceramide-induced activation of cytosolic NADH/cytochrome c electron transport pathway: An additional source of energy for apoptosis.

We have investigated whether increase in the oxidation rate of exogenous cytochrome c (cyto-c), induced by long-chain ceramides, might be due to an increased rate of cytosolic NADH/cyto-c electron transport pathway. This process was identified in isolated liver mitochondria and has been studied in our laboratory for many years. Data from highly specific test of sulfite oxidase prove that exogenous cyto-c both in the absence and presence of ceramide cannot permeate through the mitochondrial outer membrane. However, the oxidation of added NADH, mediated by exogenous cyto-c and coupled to the generation of a membrane potential supporting the ATP synthesis, can also be stimulated by ceramide. The results obtained suggest that ceramide molecules, by increasing mitochondrial permeability, with the generation of either raft-like platforms or channels, may have a dual function. They can promote the release of endogenous cyto-c and activate, with an energy conserving process, the oxidation of cytosolic NADH either inducing the formation of new respiratory contact sites or increasing the frequency of the pre-existing porin contact sites. In agreement with the data in the literature, an increase of mitochondrial ceramide molecules level may represent an efficient strategy to activate and support the correct execution of apoptotic program.

Interaction of nitric oxide with the activity of cytosolic NADH/cytochrome c electron transport system.

Nitric oxide ((.)NO) generated by the dissociation of S-nitrosoglutathione or added as gaseous solution, inhibits the oxidation of exogenous NADH supported by the activity of the cytosolic NADH/cyto-c electron transport pathway. The inhibition is immediate, very strong, higher at lower oxygen concentration, independent on the (.)NO concentration and remains constant as long as (.)NO is no more available and then is spontaneously removed. The data obtained, not in contrast with those reported with isolated cytochrome oxidase (Cox), strengthen a new concept: reduced cytochrome c (cyto-c) and (.)NO behave as two substrates of Cox, which promotes their oxidation with molecular oxygen as a co-substrate. In the presence of (.)NO, Cox exhibits the property of switching from cyto-c oxidase to (.)NO oxidase activity. With an "all or nothing" process Cox becomes an efficient (.)NO scavenger. The persistence of membrane potential, even in the presence of high inhibition of oxygen uptake, could be tentatively correlated to the protective effect of (.)NO on the ischaemic-reperfusion injury.

Stimulation by pro-apoptotic valinomycin of cytosolic NADH/cytochrome c electron transport pathway-Effect of SH reagents.

Intrinsic and extrinsic apoptosis are both characterised by the presence of cytochrome c (cyto-c) in the cytosol. We present data on the extra-mitochondrial NADH oxidation catalysed by exogenous (cytosolic) cyto-c, as a possible answer to the paradox of apoptosis being an energy-dependent program but characterized by the impairment of the respiratory chain. The reduction of molecular oxygen induced by the cytosolic NADH/cyto-c pathway is coupled to the generation of an electrochemical proton gradient available for ATP synthesis. Original findings show that SH reagents inhibit the NADH/cyto-c system with a conformational change mechanism. The mitochondrial integrity-test of sulfite oxidase unequivocally demonstrates that this enzyme (120kDa) can be released outside but exogenous cyto-c (12.5kDa) does not permeate into mitochondria. Valinomycin at 2nM stimulates both the energy-dependent reversible mitochondrial swelling and the NADH/cyto-c oxidation pathway. The pro-apoptotic activity of valinomycin, as well as to the dissipation of membrane potential, can be also ascribed to the increased activity of the NADH/cyto-c oxidation pathway useful as an additional source of energy for apoptosis. It can be speculated that the activation of the NADH/cyto-c system coupled to valinomycin-induced mitochondrial osmotic swelling may represent a strategy to activate apoptosis in confined solid tumours.

Protective effect of magnesium and potassium ions on the permeability of the external mitochondrial membrane.

The data reported are fully consistent with the well-known observation that exogenous cytochrome c (cyto-c) molecules do not permeate through the outer membrane of mitochondria (MOM) incubated in isotonic medium (250 mM sucrose). Cyto-c is unable to accept electrons from the sulfite/cyto-c oxido-reductase (Sox) present in the intermembrane space, unless mitochondria are solubilized. Mitochondria incubated in a very high hypotonic medium (25 mM sucrose), in contrast to any expectation, continue to be not permeable to added cyto-c even if Sox and adenylate kinase are released into the medium. The succinate/exogenous cyto-c reductase activity, very low in isotonic medium, is greatly increased decreasing the osmolarity of the medium but in both cases remains insensitive to proteolysis by added trypsin. In hypotonic medium, magnesium and potassium ions have a protective effect on the release of enzymes and on the reactivity of cyto-c as electron acceptor from both sulfite and succinate; results which are consistent with the view that MOM preserves its identity and remains not permeable to exogenous cyto-c. This report strengthens the proposal, supported by previously published data that in isotonic medium the exogenous NADH/cyto-c electron transport system is catalyzed by intact mitochondria, not permeable to added cyto-c.

Porin and cytochrome oxidase containing contact sites involved in the oxidation of cytosolic NADH.

Cytochrome c (cyto-c) added to isolated mitochondria promotes the oxidation of extra-mitochondrial NADH and the reduction of molecular oxygen associated to the generation of an electrochemical membrane potential available for ATP synthesis. The electron transport pathway activated by exogenous cyto-c molecules is completely distinct from the one catalyzed by the respiratory chain. Dextran sulfate (500 kDa), known to interact with porin (the voltage-dependent anion channel), other than to inhibit the release of ATP synthesized inside the mitochondria, greatly decreases the activity of exogenous NADH/cyto-c system of intact mitochondria but has no effect on the reconstituted system made of mitoplasts and external membrane preparations. The results obtained are consistent with the existence of specific contact sites containing cytochrome oxidase and porin, as components of the inner and the outer membrane respectively, involved in the oxidation of cytosolic NADH. The proposal is put forward that the bi-trans-membrane electron transport chain activated by cytosolic cyto-c becomes, in physio-pathological conditions: (i) functional in removing the excess of cytosolic NADH; (ii) essential for cell survival in the presence of an impairment of the first three respiratory complexes; and (iii) an additional source of energy at the beginning of apoptosis.